Mobile consumer devices, such as smartphones, tablets and notebook computers, continue to grow in functionality and popularity. Such devices are typically battery-powered and require chargers for charging the device batteries from a mains supply. The battery chargers are preferably small and inexpensive. Due in part to their small size, the battery chargers must be efficient so that the heat generated within them does not cause excessive case temperatures. While an individual charger draws relatively little power, the vast number of such chargers connected to a power grid potentially leads to a significant load on the power grid. Hence, regulatory agencies have developed stringent requirements for the power efficiency of battery chargers during both active and standby operational modes, so as to limit the energy wasted by such chargers. To meet these size and efficiency goals, modern battery chargers increasingly use isolated switched-mode power converters. High switching speeds used by such converters enable the transformers within the converters to be relatively small and lightweight, especially as compared with prior-generation power converters that relied on a large transformer to directly step down the alternating current (AC) mains voltage prior to rectification.
Many modern battery chargers rely upon a flyback converter topology, and variants thereof, for converting a direct current (DC) high voltage, which has been rectified from an AC mains voltage, into the low voltage needed for charging a battery. Power switches convert the high DC voltage into a high-frequency AC voltage which is applied to the primary side of a transformer. An AC voltage induced on the secondary side of the transformer must be rectified to provide the DC voltage required by a load of the converter, e.g., a battery being charged. Such rectification relies upon current-blocking devices such as diodes and/or synchronous rectification (SR) switches. SR switches are preferred for rectification in most applications, including typical battery chargers, due to their lower power losses and lower associated heat, as compared with diodes.
In addition to providing efficient power transfer while they are in active use, e.g., charging a battery, switching power converters preferably draw minimal power when there is no load attached to them. For example, consumers often leave battery chargers connected to a mains supply even after the device being charged has been removed and/or after a battery is fully charged. To minimize power loss during such standby periods, power converters often resort to a burst mode of operation. During burst-mode operation, switching of the power switches is suspended for relatively long intervals of time. Once the output voltage has decreased to a minimum allowed standby-mode voltage level during an idle interval, the switching is enabled and the output voltage is boosted to a desired maximum standby-mode voltage during a burst interval. Note that even during standby mode, leakage current drains charge from the output capacitor, necessitating occasional recharging of the capacitor through, e.g., burst-mode operation.
Burst-mode operation potentially creates a problem in that the voltage across a secondary-side rectification device, such as an SR switch, reaches much higher levels than during normal operation. Because the load (e.g., a battery) is drawing no (or negligible) current during standby/burst-mode operation, the sharp rising voltage edge of each voltage pulse on the secondary side of the transformer generates more extreme ringing of the voltage across the SR switch than occurs during normal operational modes. The upper voltage excursion may exceed a maximum allowed voltage for the SR switch. Violation of such a maximum voltage may occur repeatedly during burst-mode operation, and may damage the rectification device.
Circuits and methods are desired to limit the maximum voltage occurring across a rectification device in a switched-mode power converter during burst-mode operation. Such circuits and methods should require minimal additional circuit components, and should be power efficient.